Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the information signal into a form suitable for transmission over the channel. This processing of the information signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original information signal. This processing of the received signal is referred to as demodulation.
Two types of two-way communication channels exist, namely, point-to-point channels and point-to-multipoint channels. Examples of point-to-point channels include wirelines (e.g., local telephone transmission), microwave links, and optical fibers. In contrast, point-to-multipoint channels provide a capability where many receiving stations may be reached simultaneously from a single transmitter (e.g., cellular radio telephone communication systems). These point-to-multipoint systems are also termed Multiple Access Systems (MAS).
Analog and digital transmission methods are used to transmit an information signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of information signals, improved security of communication through the use of encryption, and increased capacity.
These advantages are attained at the cost of increased system complexity. However, through the use of very large-scale integration (VLSI) technology, a cost-effective way of building the hardware has been developed.
To transmit an information signal (either analog or digital) over a bandpass communication channel, the information signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the information signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the modulated wave matches the assigned channel bandwidth. Correspondingly, the receiver is required to re-create the original information signal from a degraded version of the transmitted signal after propagation through the channel. The recreation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter.
In addition to providing efficient transmission, there are other reasons for performing modulation. In particular, the use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several information sources over a common channel. Also, modulation may be used to convert the information signal into a form less susceptible to noise and interference.
For multiplexed communication systems, the system typically consists of many remote units (i.e., subscriber units). Each subscriber unit requires a communication channel for short or discrete intervals of time rather than continuous service on a communication channel at all times. Therefore, communication systems have been designed to incorporate the characteristic of communicating with many remote units for brief intervals of time on the same communication channel. These systems are termed time division multiple access communication systems.
A Direct Sequence (DS) Code Division Multiple Access (CDMA) system is a MAS where all subscriber units transmit information signals on the same frequency band simultaneously. Similarly, base stations transmit information signals intended for a particular subscriber unit by transmitting the information signals on the same frequency band as base station originated transmissions to other subscriber units. By necessity, the transmitted bandwidth from a base station is much larger than the information rate of the information signal, i.e. carrier bandwidth is large compared to the message bandwidth.
Most MAS use a digitized representation of voice as the information signal which must be transmitted. This type of information signal is typically produced by a speech coder. It will be appreciated by those skilled in the art that the information signal may also be derived from a data signal such as from a computer modem or data network. A base station typically accepts many information signals as input. In addition, each input signal originates from either a land based telephone or a mobile subscriber unit. Each information signal is multiplexed such that a single modified information signal is transmitted to a base station antenna at a comparatively high data rate. Various stages between the input of the information signals to antenna transmission are susceptible to noise on account of the high data rates involved and the close proximity of signal carrying conductors within the base station. The resulting effect is that capacitive or inductive crosstalk often occurs. As a result, it is desirable to achieve the highest possible utilization of the available carrier bandwidth while minimizing any errors or noise from occurring in the information signal.
Some CDMA system base stations transmit a pilot channel which aids a mobile subscriber in acquiring and tracking a voice channel. A conventional CDMA system provides the pilot channel generating function by a printed circuit board identical to the printed circuit boards used to provide voice and data channels. The output signals of each board is then converted from digital to analog signals. The signals are then combined by a functionally distinct combining circuit.
Conventional CDMA systems route the pilot channel and the voice channels to separate digital to analog converters. This introduces time skew between the pilot channel and the voice channel. If the time skew is large enough, it will create phase inaccuracies in the coherent demodulator of the mobile subscriber unit, thereby decreasing the voice channel performance.
However, even in view of the above-described conventional CDMA communication system, a need still exists for an improved digital information signal combining technique which can be applied to CDMA as well as other types of communication systems.